This invention relates to the production of ceramic articles comprising oxidation reaction products as hereinafter defined. More particularly, it relates to methods of making such articles by oxidation reaction of a parent metal with a vapor-phase oxidant, thereby to form a ceramic matrix incorporating the porous filler, and to products of such methods.
There is substantial current commerical interest in the use of ceramic materials for a wide variety of industrial, mechanical, electrical, and structural components, owing to the advantages attributes of these materials, such as their hardness, ability to withstand high temperatures, chemical inertness, beneficial properties for electrical applications, and relatively light weight. Ceramics provide attractive alternatives to metals for many existing purposes, as well as enabling the development of diverse new types of components for which metals or other materials are unsuitable.
The production of cermic components for technologically advanced applications is nevertheless attended with problems. Conventional ceramic-making methods have disadvantages including the high cost of sinterable powders, lack of batch-to-batch reproducibility of powder properties, substantial shrinkage on sintering, and susceptibility to retention of flaws produced by the forming procedure.
It is known to produce ceramics which are oxidation reaction products, viz. by reacting a precursor metal with an oxidant. As used herein, the term "oxidation reaction product" means one or more metals in any oxidized state wherein a metal (hereinafter "parent metal") has given up electrons to or shared electrons with another element, compound, or combination thereof (hereinafter "oxidant"). Accordingly, an "oxidation reaction product" under this definition includes the product of the reaction of one or more parent metals with an oxidant such as oxygen, nitrogen, halogen, sulphur, phosphorus, arsenic, carbon, boron, selenium, tellurium and compounds and combinations thereof, for example, methane, ethane, propane, acetylene, ethylene, propylene (as sources fro carbon), and mixtures such as air, H.sub.2 /H.sub.2 O and CO/CO.sub.2. Examples of suitable parent metals include, without limitation, aluminum, zirconium, titanium, silicon, zinc, hafnium, and tin.
European Patent Application No. 85301820.8, filed Mar. 15, 1985 and published Sept. 25, 1985, under publication number 0 155 831, and coassigned, describes a process for producing ceramic materials by oxidation reaction of a molten parent metal with a vapor phase oxidant, wherein the parent metal is heated to a temperature within a particular range (throughout which the parent metal is molten, but below the melting point of the oxidation reaction product) in the prsence of an atmosphere comprising or containing the vapor-phase oxidant. Formation of an oxidation reaction product occurs and proceeds with the progressive transport of molten parent metal through its own already-formed oxidation reaction product, and concomitant progressive formation of additional oxidation reaction product, thereby affording a ceramic body of advantageous thickness, with or without included unoxidated metal. In some instances, a dopant (e.g., one or more materials used in conjunction with the parent metal) may be used to enable the ceramic-forming reaction to go forward in the desired progressive manner.
It has also been known to produce ceramic composites by infiltrating a permeable bed or preform of oxidation reaction product, as adapted to the process of the above-cited European patent application. This filler material may have the same composition as the oxidation reaction product being formed or may differ in composition from the formed oxidation reaction product. A parent metal body is in extended surface contact with the permeable filler and the assembly is heated in the presence of a vapor-phase oxidant, with dopant present (where necessary or beneficial) either alloyed with the parent metal, or deposited on the surface of the parent metal body, or distributed through the filler body.
In these instances of producing ceramic composites, operating conditions are selected to achieve progressive infiltration and reaction, typically until infiltration of the permeable body with the oxidation reaction product is complete. If reaction proceeds to complete consumption of the parent metal, the produced article may be nearly all oxidation reaction product together with porosity and isolated nonoxidized metal plus any incorporated filler). If there is incomplete reaction of the parent metal, unreacted parent metal may be distributed through the produced article, and may comprise interconnected metal. The oxidation reaction product forms as a polycrystalline matrix incorporating the filler material, thereby providing a composite of the ceramic, optionally with unreacted metal and/or pores, and the filler material.
By such procedures, it is feasible to produce articles of near net shape. For instance, if a body of parent metal is surrounded by a permeable body of filler preform or particulate bed of substantially inert filler, and heated in the presence of oxidant until the metal has been completely oxidized, the resultant ceramic article will have an internal cavity conforming closely to the original external configuration of the initial parent metal body. If the permeable body itself has a defined external shape beyond which oxidation reaction product cannot occur, the produced ceramic article will have a corresponding external configuration.
With respect to these processes, it has been found that the filler may comprise particulate aggregates, wires, fibers, whiskers, woven lamina, and the like. Development work has proceeded with particulate aggregates owing to low cost and ease of making the preform. In the case of the particulate fillers, strength of the produced ceramic filler composite and many other mechanical properties are improved by reducing the particle size of the filler. However, it has been found in practice that green shapes (preforms) formed from very fine powders tend to contain forming flaws which are much larger than the maximum particle size and limit the strength of the material. Moreover, the gas permeability of the green shapes decreases with decrease of particle size, and hence the rate of the oxidation driven matrix penetration in the preform also decreases. In some cases, this oxygen starvation leads to formation of undesired constituents such as A1N which later hydrolyze and cause strength degradation.